1. Field of the Invention
The present invention relates to a fuel injection control device for an internal combustion engine.
2. Description of Related Art
Generally, a fuel injection control system of an engine calculates a required fuel injection amount based on an operation state of the engine. A fuel injection valve is driven to open with an injection pulse width (i.e., during an injection period), and the injection pulse width is set based on the required fuel injection amount. Thus, the required amount of fuel is injected.
Compared with a fuel injection valve of a port injection type engine, a fuel injection valve of a cylinder injection type engine is required to inject a large amount of fuel in a short period at a high load time, because high pressure fuel is injected into a cylinder of the cylinder injection type engine. Therefore, a large dynamic range is necessary for the fuel injection valve of the cylinder injection type engine. In this case, it is required to use a low range where a linearity of changing characteristic of actual injection amount is worse with respect to the injection pulse width. If the required fuel injection amount becomes smaller at an engine idling time, for example, a variation in the fuel injection amount becomes larger in the low range where the injection pulse width is small. Specifically, if a difference between the required fuel injection amount and an actual fuel injection amount becomes larger, exhaust emission and drivability may get worse.
JP-A-2009-513864 describes a split injection in which a required injection amount of fuel is injected separatedly between a base injection and a measurement injection. While the split injection is performed, a variation (error) in a fuel injection amount is calculated in a low range based on a difference between a target air-fuel ratio and an actual air-fuel ratio detected by a sensor. A characteristic curve representing a relationship between an injection period and an injection amount is corrected based on the calculated variation. The corrected characteristic curve is used for controlling a fuel injection valve. Thus, the variation in the fuel injection amount can be reduced.
However, if the difference between the target air-fuel ratio and the actual air-fuel ratio is generated by other factor except the variation in the fuel injection amount, an accuracy of the correction becomes low due to the other factor.
Further, under the same operation condition, a system may correct injection control characteristic based on an actual fuel-air ratio of a normal injection and an actual fuel-air ratio of a split injection, when fuel injection is performed by switching the normal injection and the split injection. However, if the operation condition is changed in the correction time, it is impossible to perform the correction, so that a predetermined frequency for performing the correction cannot be secured.